The purpose of a digital communication system is to effectively transmit and receive information over a particular channel or communication medium. To that end, the performance of any communication system is ultimately determined (and often limited) by its ability to respond to the presence of noise in the system. Digital radios often have many modes for communicating with one another, and many methods of suppressing or limiting noise within a network, such as frequency hopping, bit and frame synchronization, and numerous modulation/demodulation and filtering techniques. The Single Channel Ground-Airborne Radio System (SINGCARS) digital radio is an example of a type of radio which employs these various modes and methods of communication. In frequency hopping signal transmission systems, a wideband signal is generated by hopping from one frequency to another over a large number of frequency choices. The frequencies used are chosen by a code similar to those used in direct sequence systems. For general background on spread spectrum systems, reference is made to a text entitled Spread Spectrum Systems, 2nd edition, by Robert C. Dixon and published by Wiley-Interscience, New York (1984).
One of the most significant impairments to frequency hopping digital radio receiver operation is bias. Bias compensation is necessary since some hops can have such a large positive bias that almost all active samples can be positive. In addition, some hops can be biased in a manner wherein almost all samples in the hop could be negative. The ideal received waveform has an eye pattern, where every "0" is positive and every "1" is negative. The eye pattern is symmetrical about the x axis so that the opening in the eye pattern, at bit sampling time, has equal margin for zeros and ones. A systematic bias occurs when some disturbance in a radio, such as those caused by hardware transients during frequency transitions, causes a constant to be added to every sample in a hop. If this constant is positive, then this reduces the margin of the negative values and may cause errors even in the absence of noise; if this constant is negative, it reduces the margin on the positive values. This represents a major problem for almost any communication system, but is particularly troublesome for military applications and emergency situations, where constant communication and informational updates are vital to mission success. Therefore, providing a method for bias compensation is desirable not only for improving bit decisions but also for enhancing system timing and enabling transmitter/receiver synchronization, since these processes are dependent upon timely and accurate detection of a signal in the presence of noise.
However, trying to estimate the systematic bias can result in the detection of a data dependent bias caused by a variation in the number of zeros relative to the number of ones in a hop which is unrelated to the true center of the eye pattern. It is then when the estimated bias is removed, which is affected by the data pattern, that systematic bias may be introduced rather than mitigated. Consequently, an improved method for estimating and removing bias that is relatively insensitive to data pattern variations, robust in noise, and operable in a frequency hopping digital communication system is greatly desired.